In a motor-vehicle engine system, fuel from a fuel tank is intended to flow to the combustion chambers of the engine with unit efficiency, such that no fuel is released into the atmosphere. In practice, various measures are taken to recapture fuel that has escaped its intended flow path and might otherwise be released into the atmosphere as vapor. Such fuel is typically redirected to the intake manifold of the engine.
For instance, a positive crankcase ventilation (PCV) system may be used to recapture and combust fuel vapor that has entered the crankcase. In addition, fuel vapor vented from the fuel tank (whether the motor vehicle is operating, resting, or being refueled) may be temporarily trapped in an adsorbent canister and delivered to the intake manifold during a subsequent purge of the adsorbent canister. In motor-vehicle engine systems used today, the crankcase and adsorbent canister, maintained near atmospheric pressure by coupling to an air cleaner, may each communicate with the intake manifold via a control valve. The vacuum that may be present at the intake manifold provides a motive force to draw fuel vapor from the crankcase and/or adsorbent canister and into the engine, where it is combusted.
Crankcase ventilation and fuel-vapor purging as described above may be effective and reliable so long as sufficient vacuum is available at the intake manifold. In boosted engine systems, however, sufficient vacuum may be unavailable during some operating conditions, such as during medium- or high-level boost. One solution to this problem is to provide a supplementary source of vacuum such as an electrically driven vacuum pump to purge fuel vapor from the crankcase and/or adsorbent canister when intake manifold vacuum is not available. However, this approach increases engine-system cost and complexity.
Alternative approaches that provide adsorbent-canister purging driven by positive pressure instead of manifold vacuum have appeared (e.g., U.S. Pat. No. 7,284,541). However, this approach may not provide effective purging when the manifold air pressure is close to barometric pressure. Further, it maintains the fuel tank at less than barometric pressure during some operating conditions, potentially increasing the rate of emission of vapor from the fuel tank and the required purge frequency. Therefore, one embodiment provides a method for combusting a vapor of a fuel accumulated in an adsorbent canister in a vehicle, the vehicle having an engine and a fuel tank coupled to the adsorbent canister. The method comprises, during a first operating condition including a higher availability of compressed air, metering the compressed air from a first air source, and flowing the metered, compressed air through the adsorbent canister along a first flow path. The method further comprises, during a second operating condition including a lower availability of compressed air, flowing air from a second air source through the adsorbent canister, and, during the first and second operating conditions, venting the fuel tank to an intake of the engine and flowing effluent enriched in the vapor from the adsorbent canister to the intake of the engine.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.